In radio frequency (RF) transmission systems, filters and switches are commonly used. Filters are used to filter out signals having unwanted frequencies and only allow signals in a desired frequency range to pass through with a minimum of attenuation. Switches at the input and output ports of the filter are used to effect coupling amongst, for example, a transmitter, receiver and antennas of a system.
Various type of filters are used, such as high pass filters, low pass filters, band stop filters and band pass filters. Normally a band pass filter is used between antennas and a transmitter or a receiver of a system so that only signals in a desired band of frequencies passes through the filter. A typical input-output amplitude relationship 100 of a bandpass filter is shown in FIG. 1, where the horizontal axis 105 represents frequency and the vertical axis 110 represents the amplitude response H(f) of the filter.
As shown in FIG. 1, the bandpass filter having the input-output amplitude relationship 100 has a passband between cut-off frequencies f.sub.1 115 and f.sub.2 120 and centered around a center frequency f.sub.c 125. The difference between f.sub.1 115 and f.sub.2 120 is often referred to as the bandwidth of the filter. The center frequency f.sub.c 125 is the frequency of operation of the system and is often referred to as the resonant frequency f.sub.0 which is given by equation (1). ##EQU1##
The resonant frequency is defined as that frequency at which the inductance resonates with the capacitance. This occurs when the inductive and capacitive reactances .omega.L, 1/.omega.C are equal in magnitude, or ##EQU2##
A Quality factor Q is often associated with resonant circuits which defines the roll-off rate of the response curve 100 which often results in a higher amplitude at the center frequency f.sub.c 125 and a narrower bandwidth. The Quality factor Q is a function of the energy stored in the inductor and capacitor of a resonant circuit and the energy lost in the resistor. For example, the series LC circuit 200 shown in FIG. 2a has a Q defined by equation (5) ##EQU3##
Similarly, the LC parallel circuit 250 shown in FIG. 2b has a Q defined by equation (6) ##EQU4##
To reduce the attenuation of desired signals passing through the filter, a matched filter is often used, particularly in radar and digital data systems. The matched filter is designed to have nearly the same transfer characteristic as the source of the inputted signal. Ideally, the signal output by the matched filter is the same as the inputted signal (except for the total absence of the portions of the inputted signal which were filtered out) and is merely delayed in time by t.sub.0. The amplitude of the input signal having frequencies in the passband remains relatively unchanged as the input signal goes through the filter whereas, for frequencies outside the passband, the amplitude is entirely attenuated by the filter.
The matched filter is designed to have an impedance at its ports which matches the impedance of the subsystems attached to the filter's ports. Impedance or admittance transformer circuits are used to convert the input/output impedances or admittances of the filter to a desired value. The desired value of the input/output impedance or admittance of a matched filter is the complex conjugate of the impedance or admittance of the external circuits connected to the filter.
Basically, a filter can be considered as a combination of n number of resonators (e.g., reactive element 310, 360) and n-1 number of impedance or admittance transformers 305, 355 as shown in FIGS. 3a and 3b.
FIG. 3a shows a generalized band-pass filter circuit 300 having 1 to n stages. Each stage of the band-pass filter circuit 300 has an impedance transformer 305 connected to a reactive element 310. The impedance transformers 305 are designed to convert the input impedance Zin 315 of the filter 300 to match the input resistor R.sub.A 320. The input resistor R.sub.A 320 is the impedance of a purely resistive system connected to the input of the filter 300. Illustratively, the impedance of a system connected to the input of the filter 300 has a value of 50 .OMEGA..
Instead of a purely resistive system having the input resistor R.sub.A 320, the system connected to the band-pass filter circuit 300 may have a complex impedance Z.sub.A. A filter designed to match such a system, will have an input impedance Zin 315 equaling Z.sub.A * which is the complex conjugate of input system's impedance Z.sub.A. The input impedance Zin 315 is the impedance looking into the input terminals 325 of the filter 300. Similarly, the output impedance Z.sub.out 330 is the impedance looking into the output terminals 335 of the filter 300. The impedance transformers 310 also convert the output impedance Z.sub.out 330 to match the output resistor R.sub.B 340 for systems connected to the output of the filter 300 having a purely resistive impedance. The impedances looking into each stage 1 to n may be different; therefore, a different impedance transformer 310 is illustratively used for each stage.
Similarly, FIG. 3b shows a generalized band-pass filter circuit 350 using admittance transformers 355 connected between reactive elements 360. The admittance transformers 355 convert the input and output admittances G.sub.in 365, G.sub.out 370 of the filter 350 to match the input and output conductances G.sub.A 375, G.sub.B 380 respectively. As discussed above in connection with impedance transformers, for a system connected to the input of the filter 300 having a complex admittance Y.sub.A, the admittance of a matched filter 350 is Y.sub.A * which is the complex conjugate of input system's admittance Y.sub.A.
At the resonant frequency, the converted input/output impedances Z.sub.in 315, Z.sub.out 330 or admittances G.sub.in 365, G.sub.out 370, match the input/output resistors R.sub.A 320, R.sub.B 340 or conductances G.sub.A 375, G.sub.B 380. Such matching provides for a maximum power transfer between the input and output of the filter and produces a minimum distortion in the signal passing through the bandpass filters 300, 350.
In the passband, for example at the resonant frequency (f.sub.0 or .omega.=.omega..sub.0), the impedance of the reactive elements 310 have a small susceptance value (i.e., nearly a short circuit) and the signal passes through the filter 300 with a minimum distortion. Similarly, in the passband, the admittance of the reactive elements 360 have a large susceptance value at resonance (i.e., nearly an open circuit) and very little of the input signal is shunted therethrough. Instead, the input signal passes through the filter with a minimum attenuation. Similar to the input and output impedances of FIG. 3a, the input and output admittances of each stage 1 to n of FIG. 3b are different requiring different admittances transformers 355 for each stage.
FIG. 4 shows a symmetric nine stage filter 400, having an impedance Z.sub.0 (f) 405 looking into the terminals 410 of the first stage 415 which is the same as the impedance looking into the terminals 420 of the ninth stage 425. The impedance Z.sub.1 (f) 430 looking into the terminals 435 of the third stage 440 is equal to the impedance looking into the terminals 445 of the seventh stage 450. Similarly, the value of the impedances looking into the terminals 455 and 460 of the fifth stage 465 are the same and equal to Z.sub.2 (f) 470.
FIG. 5 shows a two port bandpass filter 500 comprising a five stage lumped bandpass filter. The two port bandpass filter 500 has two series LC filters 502 and 504 (which are the second and fourth stages respectively) and three parallel LC filters 506, 508 and 510 (which are the first, third and fifth stages respectively). The first port 512 is connected to a node 514 which is connected to the series LC filter 502 and the parallel LC filter 506. The parallel LC filter 506 has a capacitor C.sub.5 516 having a first terminal 518 connected to the node 514 and a first terminal 520 of a inductor L.sub.5 522. A second terminal 524 of the capacitor C.sub.5 516 and a second terminal 526 of the inductor L.sub.5 522 are grounded.
The node 514 is also connected to a first terminal 528 of an inductor L.sub.4 530. A second terminal 532 of the inductor L.sub.4 530 is connected to a first terminal 534 of a capacitor C.sub.4 536. A second terminal 538 of the capacitor C.sub.4 536 is connected to a node 540.
A parallel LC filter and a series LC filter are connected to node 540 in a similar fashion as the connection to node 514. That is, a capacitor C.sub.3 542 and an inductor L.sub.3 544 are connected in parallel, with one side 546 connected to the node 540 and another side 548 connected to ground.
An inductor L.sub.2 550 is connected in to a capacitor C.sub.2 552 to form the series LC filter 504. One side 554 of the series LC filter 504 is connected to the node 540 and another side 556 is connected to a node 558. Also connected to the node 558 is the second port 560 and the parallel LC filter 510. The parallel LC filter 510 is formed by a capacitor C.sub.1 562 and an inductor L.sub.1 564.
The frequency response 600 of the filter 500 is shown in FIG. 6, where the value of the components of the filter 500 are as follows:
______________________________________ L.sub.1 = L.sub.5 = 2.1 nH C.sub.1 = C.sub.5 = 18 pF L.sub.2 = L.sub.4 = 33 nH C.sub.2 = C.sub.4 = 1.2 pF L.sub.3 = 1.5 nH C.sub.3 = 27 pF ______________________________________
The horizontal axis 605 shown in FIG. 6 represents frequency from 450 MHz to 950 MHz in increments of 50 MHz. The vertical axis 610 represents magnitude of the output of the filter 500 of FIG. 5 in response to an input signal having a 0 dB amplitude from 450 MHz to 950 MHz. The vertical axis 610 is in 10 dB increments starting from 0 db at the top and going down to -100 dB. The center frequency, where an arrow 615 is pointing, is 697 MHz and the magnitude of the frequency response 600 is -5.2399 db.
Switches are often connected to the input and output ports of a filter to selectively interconnect the filter ports. In RF transmission systems, for example, switches are connected to filter ports to selectively interconnect the transmitter, receiver and antenna of the system. U.S. Pat. Nos. 4,701,724, 4,803,447 and 5,023,935 disclose such a combination of a filter connected externally to switches. The '724 and '935 patents disclose a first and second coupled quarter-wave transmission lines, acting as bandpass filter, and switches or diodes externally connected to the bandpass filter.
In all three patents '724, '447 and '935, a first series PIN (p-type, intrinsic silicon, n-type) diode switch element is connected to an input terminal of the filter and a second PIN diode switch shown is connected in parallel to another terminal of the filter. Because there is no impedance transformers to convert the impedance of these PIN diodes to a high impedance, the insertion loss contributed by these diodes is large. In addition, the second PIN diode switch shown in the three patents are connected in parallel to one of the ports of the filter. In all three patents '724, '447 and '935, the filters are independent modules similar to a block diagram 700 shown in FIG. 7a. That is, these filters are designed independently from the switches, and later integrated with the PIN diode switches. Furthermore, the filters disclosed in patents '724 and '447 are narrow band filters tuned to a single frequency.
FIG. 7a shows a block diagram 700 of a transceiver having a filter and switches similar to the ones disclosed in '724 and '935. More particularly, instead of a single antenna used in analog radio systems, two antennas are used in a digital radio system as shown in FIG. 7a. The block diagram 700 of FIG. 7a has become a standard module in an ever more popular TDMA (Time Division Multiple Access) digital wireless communication systems. The block diagram 700 used in digital radio systems has two separate single pole double throw (SPDT) switches 705, 710 and a bandpass filter 715.
The block diagram 700 has four ports which are the ports of the two SPDT switches 705, 710. A first port 720 is connected to a receiver 725 while a second port 730 is connected to a transmitter 735. The third and fourth ports 740, 745 are connected to a first and a second antenna 750, 755 respectively. The bandpass filter 715 is connected between a center terminal 760 of the first SPDT switch 705 and a center terminal 765 of the second SPDT switch 710. The two SPDT switches 705, 710 interconnect the two antennas 750, 755 to the receiver 725 and the transmitter 735 through the bandpass filter 715.
In the default condition, shown in FIG. 7a, the first antenna 750 is connected to the receiver 725. The two antennas 750, 755, the receiver 725 and the transmitter 735 can be connected in any combination by switching the two switches 705, 710.
For a system having an impedance of Z.sub.0, the impedance of a matched filter should also be Z.sub.0. Thus, the impedance looking into the filter 715 is Z.sub.0 as is the impedance looking into the systems connected to the switches 705, 710.
The block diagram 700 of the digital radio system is commonly used in communication systems, modems and cellular telephones. Having the two independent SPDT switches 705, 710 connected to the bandpass filter 715 contributes to losses in the system. These additional losses results in a high loss system which pose stringent restraints and requirement in the design of the receiver, transmitter and antennas connected to the switches 705, 710 and the filter 715.
FIG. 7b shows a small signal model of the dotted portion 760 of the block diagram 700 of FIG. 7a. The switch 705 of FIG. 7a is represented by its small signal resistance r 765. The resistance r 765 is connected between two impedances Z.sub.0 770 representing the impedances of the receiver 725 and filter 715 connected to the switch 705.
The amplitude in dB of the theoretical loss of each of the two SPDT switches 705, 710 of FIG. 7a is given by equation (7) below: EQU .alpha..sub.0 =201og(1+g.sub.F /2) dB (7)
where, EQU g.sub.F =r/Z.sub.0 (8)
where Z.sub.0 is the impedance of the system and has a value of 50 .OMEGA., and r is the small-signal resistance of the switch 705.
The block diagram 700 can be implemented by using the filter 500 shown in FIG. 5 as the bandpass filter 715 of FIG. 7a. The two SPDT switches 705, 710 can be implemented by using PIN diodes. Illustratively, an HSMP-3890 PIN diode made by Hewlet Packard can be used. The HSMP-3890 PIN diode is biased with approximately a 5 mA bias current and has a dynamic resistance of approximately 5 .OMEGA.. Based on experimental measurements and according the equation (7), the loss per conducting PIN diode is found to be at least 0.4 dB. Because there are two SPDT switch, two PIN diodes conduct at any one time. Therefore, using PIN diodes and the filter 500 of FIG. 5 to implement the block diagram 700 having the two SPDT switches 605, 610 results in a filter that has a loss of at least 0.8 dB due to the two SPDT switches. This 0.8 dB loss is in addition to the 5.2399 dB loss of the filter 500 (FIG. 5) at 697 MHz as shown in the amplitude response of FIG. 6.
Such losses cause the system 700 to suffer from a low efficiency, as half of the transmit or receive power may be lost. Losses of power closer to the antenna are more significant causing fading of the received signal. In addition, such losses reduce the power of the signal transmitted from the antenna to inadequate levels. Such losses are particularly significant for battery operated portable machines, such as cellular telephones and personal communicators, where the excess power required to compensate for the losses discharges the battery quickly.
To compensate for the losses, not only a larger battery is needed, but also larger components may be needed, such as larger transmitters and antennas. The additional size and weight are undesirable and a serious drawback to portable equipment, where the trend is to miniaturize as much as possible. In addition, highly complex and costly circuitry may be needed in the receiver and/or the transmitter, such as low noise high gain amplifiers and high sensitivity receivers.
It is therefore an object of the present invention to provide a switch filter which overcomes the disadvantages of the prior art. It is another object of the present invention to provide a multi-port switch filter which provides filtering and switching, operates efficiently, consumes less power and has a low insertion loss. It is a further object of the present invention to reduce the size of the switch filter.